Signaling system



' April 26, 1949. N. A. NEWELL SIGNALING SYSTEM 4 sheets-sheet 1 Filed NOV. 1, 1946 IM/ENTOR N- A. AEWELL 0% 6% ATTORNEY April 26, 1949.

Filed Nov 1, 1946 N. A. NEWELL SIGNALING SYSTEM 4' Sheets-Sheet 2 FIG. 2'

283 IFFEkiNT/AL INVENTOR N. A. NE WELL A TTORNEV N. A. NEWELL SIGNALING SYSIEI April 26, 1949. I

Filed Nov. 1,. 1946 5 4 Sheets-Sheet 4 INVENTOR N. A. AEWELL ATTORNEY mbv Patented Apr. 26, 1949 SIGNALING SYSTEM Norman A. Newell, Mlllburn, N. J assignor to Bell Telephone Laboratories,

Incorporated, New

York, N. Y., a corporation of New York Application November 1, 1946, Serial No. 707,255

Claims. 1

This invention relates to signaling systems and particularly to systems in which signaling is ef-' fected by the transmission of alternating currents.

Objects of the invention are the provision of more reliable signal receiving means for response to alternating current signals and the prevention of false operation of the signal receiving means responsive to interference currents having components of the particular frequency or frequencies used for the transmission of signals.

This invention is a signaling system in which the termination of currents of a particular frequency normally transmitted over a line constitute the first signal, in which an impulse of current of said particular frequency constitutes a second signal and in which the continued transmission of current of said particular frequency constitutes a third signal. A feature of the invention is a signal receiver comprising voltage dividing networks tuned to the signal frequency and connected through uni-directional devices to differentially energize the windings of a signal relay and effect operation of the relay only when current of the signaling frequency predominates.

Another feature of the invention is a signaling and switching system in which a signal receiver located at an intermediate switching point in the system is arranged to respond to signals transmitted over a line by current of a particular frequency and comprises means for connecting a band elimination filter unit in the line and bridging a band-pass filter unit across the line when the signal receiving relay is operated thereby to prevent interference from echoes and to substantially prevent the transmission of the signal current beyond the switching point.

A clear and complete understanding of the invention will be facilitated by considering a system embodying the invention and its features, one such system being represented schematically in the drawing of this specification. The invention is not limited in its application to the system and circuit arrangements shown in the drawings but is applicable generally to any system in which signals are transmitted by voice frequency alternating currents.

Referring to the drawings:

Figs. 1 and 2 represent a first toll omce comprising an operator position OPI, an outgoing trunk OTI, a two-way trunk circuit TWTI, a selector TSI, an incoming trunk circuit ITI, a signal transmitting circuit STI, and a signal receiving circuit SRI;

Figs. 3 and 4 represent a second toll oillce com- 2 prising an operator position 0P2, an outgoing trunk GT2, a two-way trunk circuit TWT2, a selector T82, an incoming trunk circuit ITZ, a signal'transmitting circuit STZ, and a signal receiving circuit SR2; and

Fig. 5 shows the relative positions of Figs. 1 to 4 to form an operative arrangement.

The system represented in the drawings includes a plurality of toll offices, each of which comprises a toll switch board with operators positions and cords for answering calls incoming from manual or dial local ofllces, from toll subscribers lines, from community dial ofllces, or from other toll ofllces; and for connecting calling intertoll trunks with switching trunks leading to called local manual or dial oillces, to called toll subscribers lines, to trunks to community dial offlces, and to intertoll trunks to other toll oilices. Outgoing jacks are connected to switching trunks, toll subscribers lines, trunks to community dial oiiices and intertoll trunks. Answering jacks are connected to recording trunks, toll subscribers lines, trunks from community dial oifices and intertoll trunks. Intertoll first selectors, and intertoll second selectors, if required, are provided for use on calls incoming over intertoll trunks to establish connections with other toll, local or community dial omces. The operators positions are provided with dials for use in controlling the operation of intertoll selectors in other toll oflices and for controlling the operation of selector and connector switches in local or community dial oilices.

Reference may be had to the patent to R. E. King and O. R. Miller No. 2,209,777 granted July 30, 1940 and to the patent to J. G. Walsh No. 2,306,236 granted December 22, 1942 for a complete disclosure of cord and operator position circuits provided in each of the two toll oillces for interconnecting calling and called lines and trunks and controlling the completion of toll calls. The outgoing trunk circuits OTI and GT2, incoming trunk circuits ITI and 1T2, two-way trunk circuits TWTI and TWTI, and the toll route selector circuits TS! and T82 are all similar to corresponding circuits of the aforementioned King-Miller and Walsh patents.

The two toll omces shown in the drawings are interconnected by four-wire lines, one being shown in the drawings as consisting of two, twowire lines TLI and TL! associated with trunk circuits OTI and TWTI in the first toll oflice and with trunk circuits 0T2 and TWT2 in the second toll oflice. The line TLI is used to transmit voice and voice frequency signaling currents from the first toll ofllce to the second and the line TL2 is used to transmit voice and voice frequency signaling currents from the second toll office to the first. A hybrid coil HC! individual to the trunk circuits OT! and TWT! is arranged to transmit voice currents from these trunk circuits over toll line TLI to the second toll office; and voice currents incoming over line TL2 are transmitted through repeating coil 228, a one-way amplifier 248 and hybrid coil HC! to the trunk circuits. A like hybrid coil HC2 individual to the trunk circuits T2 and TWT2 is arranged to transmit voice currents from the trunk circuits over toll line TL2 to the first toll offlce; andvoice currents incoming over toll line TL! are transmitted through repeating coil 328, one-way amplifier 348 and hybrid coil HC2 to these trunk circuits. Balancing networks BNI and BN2 are connected to hybrid coils HC! and H02 in usual and well-known manner.

The signal transmitter ST! comprises a generator 2!8 which generates alternating current of a particular voice frequency, for instance 3,200 cycles per second, a relay 288 responsive to seizure, selective, supervisory and disconnect signals which are to be transmitted over the toll line TLI an auxiliary relay 286 and a cut-oil relay 285 for disconnecting the toll line TL! from the hybrid coil HC! during the transmission of signals outgoing over toll line TLI. The generator H8 is normally connected through resistors 2!! and H2, resistors 2l3 and 2!4, back contacts of relay 288, conductors 2!! and 2l8, and windings of repeating coil 2!6 to the conductors of toll line TLI, the continued presence of a tone of 3,200 cycles on the toll line indicating that the line is idle. A one-way amplifier 248, connecting the toll line TL2 to the hybrid coil HCI, prevents the transmission of signaling current from signal transmitter ST! over toll line TL2.

The signal transmitter ST2 associated with toll line TL2 in the second toll oflice is similar to the signal transmitter STI, corresponding elements having reference characters with the same tens and units digits. The signaling current from generator 3l8 is normally transmitted over toll line TL2; and the one-way amplifier 348 is eifective to prevent the transmission of signaling current from generator 3!8 over toll line TLI.

The signal receiver SR! comprises an input transformer 258 which is connected to the conductors of toll line TL2 in parallel with the primary winding of repeating coil 228 through which coil voice currents are transmitted to the input side of amplifier 248. The signal receiver SR! further comprises a thermionic amplifier 256, an output transformer 258, a voltage dividing network 268, 'full wave rectifiers 218 and 288, a signal receiving relay 298 and an auxiliary relay 29!. The resistor'255 in the grid circuit of tube 256 is a high resistance so that the signal receiver constitutes a high impedance shunt across the talking channel. Resistor 255 is also effective to limit the voltages applied to the grid of tube 256 due former 258, consists of two tuned circuit unitsconnected in series, one unit being parallel resonant and the other series resonant at the signaling frequency. The parallel tuned circuit unit consists of condenser 26! and coil 292 connected in parallel, and the series tuned circuit unit consists of condenser 269 and coil 264 connected in series. A full wave rectifier 218 comprising copper-oxide type rectiflers 21!, 212, 219 and 214 is connected across the parallel resonant circuit unit and a full wave rectifier 288 comprising rectiflers 28!, 282, 283 and 294 is connected across the series resonant circuit unit of network 268. One winding of the signal receiving relay 298 is connected across the output terminals of rectifier 218 and the other winding of relay 298 is connected across the output terminals of rectifier 288 in such a manner that the windings are differentially energized. Two unidirectional conducting elements 289 and 289 are provided for shunting the winding which is connected to the series resonant circuit unit so as to prevent the operation of relay 298 when the current incoming over toll line TL2 is predominantly of frequencies other than the particularfrequency used for signaling. When signaling current is being received over toll line TL2, the parallel resonant circuit unit 26!, 262 constitutes a very high impedance and the series resonant circuit unit 263, 264 constitutes a low impedance; whereby the voltage applied across rectifier 218 is high and' the voltage applied across rectifier 288 is low. so that the energization of the left winding of relay 298 exceeds the energization of the right winding of relay 298 sunlciently to effect the operation of this relay. When the current incoming over line TL2 consists of both signaling and non-signaling frequencies, which produce equal voltages across the parallel and series tuned circuits of network 268, the voltages appiied through rectifiers 218 and 288 to the windings of relay 298 are equal and the relay 299 thus remains deenergized. When non-signalling frequencies predominate in the current incoming over line TL2, the impedance of the parallel resonant circuit unit is low andthe impedance of the series resonant circuit unit is high; but due to the aforementioned shunting of the right windingof relay 298 by unidirectional elements 288 and 299, the energization of the right winding in one direction does not exceed the energization of the left winding in the opposite direction sufficiently to cause operation or the relay.

Since signaling current is normally transmitted over line TL2, relay 298 is normally operated; and so also are auxiliary relays 29! and 238. Relay 29!, when operated, disconnects the parallel .resonant circuit unit of network 268, short circuits the series resonant circuit unit of network 268, bridges resistor 294 across the secondary winding oftransformer 258, and closes a circuit through resistor 293 for discharging condenser 292. Thus with relay 29! operated, the full voltage developed across the secondary winding of transformer 259 is applied across the resistor 294 and rectifier 218 energizing only the left winding of relay 298; so that the superimposing of voice currents or any interference currents of voice frequency on line TL2 is ineffective to'cause the release of relay 298; and relay 298 remains operated until the transmission of signaling current over line TL2 is terminated. Relay 29! is rendered slow to operate by the connection of due to the charging of condenser 292 is long enough so that relay 29I does'not operate when relay 290 is being alternately operated and released responsive to dial impulses as hereinafter described. It is also the function of relay 29I to prevent release of relay 290 by voice currents transmitted over line TL2 from an answering operator in case the call is intercepted. To prevent interference from echoes and to prevent the transmission of signals incoming over to a line TL2 through amplifier 240, coil HCI, trunk circuits OTI and TWTI and out over another toll line on a built-up connection, the operation of relay 290 connects a parallel resonant circuit unit consisting of condenser 23I and coil 232 in series with the secondary winding of repeating coil 220 and bridges a series resonant circuit unit consisting of condenser 233 and coil 294 across the secondary winding of repeating coil 220. These circuit units are resonant at the signaling frequency,

so little if any current of signaling frequency is transmitted through the voice channel while relay 230 is operated. Thus, signals incoming over toll line TL2 cannot be transmitted through the talking path to any other line with which trunk circuit T2 may 'be connected. Furthermore, very little of the tone signal incoming over line TL2 is transmitted through amplifier 240 and hybrid coil HCI back over toll line TLI and the possibility of false operation of the signal receiver SR2 by echo effects is substantially prevented.

The signal receiver SR2 in the second toll oifice is similar to the signal receiver SRI and the reference characters applied to corresponding elements have the same tens and units digits. Since signaling current is normally being transmitted from generator 2I0 of signal transmitter STI through repeating coil 2I6, over toll line TLI, relays 390, 39I and 330 are normally operated in similar manner to relays 290, 29I and 230 of signal receiver SRI as above described and with like effect.

Assume now that the plug API of the cord CDI at position OPI in the first toll oilice is inserted in an answering jack to answer a call incoming over a recording trunkor an intertoll trunk or a trunk from a community dial office, that the operator has determined the destination of the call, that the plug CPI of this cord is inserted in a jack associated with an outgoing trunk circuit over which the call may be extended, and that the trunk OTI is associated with the jack J I into which the plug CPI is inserted. The insertion of plug CPI in jack J I closes a circuit for operating relay I04, thereby causing the operation of relay IIO as described in the aforementioned King et al. patent and closing a circuit including conductor I02 for operatively energizing the upper winding of signaling relay I40. The operation of relay I I0 disconnects the signaling conductor I10 from conductor I69 thereby to prevent the operation of relay I003 of two-way trunk circuit TWTI when ground is connected to signaling conductor I10, connects answering supervisory relay I05 of the trunk circuit OTI to conductor I10, and connects ground to conductor I59 to operate relay I004 of two-way trunk circuit TWTI. Relay I004 locks to conductor I69 independent of relay I003 and further opens the connection between the winding of relay I003 and the signaling conductor I10. The operation of' relay I40 connects signaling battery through resistor I42, to conductor I64 to operatesignal relay 200 of the signal transmitter STI. The 012-- eration of relay 200 terminates the transmission ofsignaling current of 3200 cycles per second from the generator 2 I 0 over toll line TLI thereby causing the. release of the normally operated relay 390 of signal receiver SR2. closes a circuit through its inner lower front contact for operating cut-01f relay 205. Relay 205 disconnectsthe repeating coil 2I6 of toll line TLI from conductors 20I and 202 leading to the hybrid coil HCI, connects the repeater termination comprising resistor 209 and condenser 204 across the repeating coil 2I6, connects ground to conductor I68 to hold relay I004 operated independent of relay H0, and closes a circuit for operating relay 205. Relay 206 locks under the sole control of relay 200, opens the operating circuit of relay 205, and opens the shunt circuit through resistor 2I5 across the generator 2I0 thereby to increase the voltage applied to conductors 2 and 2| 8 when relay 200 releases as hereinafter described. The release of relay 205 disconnects the repeater termination and closes the talking connection between repeating coil 2I6 and hybrid coil HCI.

The termination of the signaling current normally transmitted over toll line TLI eifects the release of relay 390 of signal receiver SRI. The release of relay 390 connects ground to signaling conductor 410 to cause the operation of relay 503 of trunk circuit TWT2. The circuit for operating relay 503 is traced from ground at the back contact of relay 990, through conductor 410, lower' back contact of relay4l0 of trunk circuit 0T2, conductor 469, back contact of relay 504 of trunk circuit TWT2, back contact of relay 501 and the winding of relay 503. ,Relay 503 connects ground to conductor 469, to guard the outgoing trunk circuit 0T2 from being seized on an outgoing call, and closes a circuit for operating relay 505. This circuit is traced from ground at the back contact of relay 390, through conductor 410, lower back contact of relay 4I0 of outgoing trunk circuit 0T2, conductor 469, back contact of relay 504 of two-way trunk circuit TWT2, .winding of relay 505, uppermost front contact of relay 503, conductor 461, thence in simplex through resistors 42I' and 422, back contacts of relay 420 of outgoing trunk circuit 0T2, windings of retard coil 460, conductors 46I and 462, inner back contacts of relay 430, conductors 50| and 502, front contacts of relay 503, back contacts of relay 560 of selector TS2, resistors 56I, 562 and 569, and the winding of line relay 551. The line relay 551 of toll selector TS2 is thereby operated and so also is relay 505 of th two-way trunk circuit TWT2. Relay 505 closes a circuit through resistor 506 for holding relay 509 operated and closes a circuit for operating the slow-to-release relay 501. Relay 501 also closes a circuit through resistor 506 for holding relay 503operated, opens the operating circuit of relay 503 and connects ground to conductor 553 of selector TS2 to hold selector TS2 (and any additional selector through which the connection is extended in the second oflice) until the connection is released at th first toll ofllce.

When the calling operator in the first toll oiilce operates the dialing key, (not shown) the upper winding of relay I40 is controlled by the impulse contacts of the dial at position OPI. Each release and reoperation of relay I40, in

response to the dialing of the first digit of thecalled number, opens and closes the circuit through conductor I64 and the winding of relay 200. causing a corresponding release and re.

Relay 200 also trunk circuit ITi terminates at jack J l.

assasos operation of relay 2. When may in mm responsive to the iirst impulse of the digit, generator ill is reconnected through resistors ill,

. ili, ill and ill. back contacts of relay ill and conductors ill and ill to the repeating coil ill; and 3200'cycle current is'transmitt'ed o'ver toll line TLI during the interval that relay ill is released. This current is amplified by tube ill of signal receiver SM and eflects the operation of relay ill. When-relay ill reoperates at .the end of the first dial impulse. generator ill is disconnected from conductors ill and ill thereby terminating thecurrent or the signaling i'requency'to eflect the release of relay lll of signal receiver SR2. Thus. each release and reoperation of relay Ill in response to the dialing of trunk circuit 'rw'r: and line relay 551 or selector TSi are alternately released and 'reoperated a like number of times. Since relay I" is slow in releasing, the response of relay Ill to dial impulses is ineffective to cause the release of relay 5". Each release of relay 551 causes the operation of the vertical stepping magnet (not shown) of selector TSi whereby the brushes 5', 512, ill and I'll are stepped up to the level corresponding to the digit dialed. At the end of this train of impulses, the brushes are advanced step-by-step in the selected level until a set of terminals connected to an idle trunk or succeeding selector is encountered: whereupon relay ill is operated to extend a connection to the selected switch or trunk all in usual and well-known manner.

Each succeeding train of dial impulses created by the operation of the calling operator's dial is repeated by relay Ill to relay ill; and a corresponding train of impulses of voice frequency signaling current is transmitted over toll line TLI to operate relay ill. The operation of relay lll, responsive to these trains of impulses, effects the transmission of corresponding trains of impulses through brushes ill and 512 and the selected terminals of selector TSi to operate succeeding switches through which the connection is extended or to operate digit registers. When dialing is completed, the dial key at position,

OPI is restored to normal and relay Ill is operated in the manner described in the aforementioned King-Miller and Walsh patents to complete the talking connection between cord CDI and hybrid coil HCIL Assume that the call in question is one which v completed through condensers lll and lil.

8 back contacts of relay lil, windings oi retard coil lll, conductors lll and lli, back contacts of relay lil. conductors Ill and Ili, front contacts of relays Ill and Ill, brushes Ill and 512 of selector TSi, conductors Ill and lli of trunk circuit I'li, windings oi retard coillll, resistors lll, lll and Ill and the winding of line relay lll. Relay lll is operated by the current in this circuit thereby to hold relay Ill and thus maintainthe connection of ground through conductor lll. Relay lll closes a circuit for lighting lamp Ll. When a plug of the cord CDi at position OH is inserted in jack J l to answer the call, the lamp Ll is extinguished and the talking connection is Relay lll is operated in the manner described in the arorementioned patent to J. G. Walsh and closes a circuit from ground through conductor lll,

' brush ill of selector TSi, a front contact of relay Ill, and the winding of relay lll oi two-way trunk circuit TWTi; The operation of relay lll closes a circuit for operating relay ill of S nal transmitter STi. This circuit'is traced from battery through resistor, i, conductor 468, front contact of relay ll I, back contact of relay lll, conductor lll, back contact of relay 0, conductor lll and the winding of relay ill. The operation of relay ill disconnects generator ill from conductors an and Ill to terminate the transmission of signaling current over toll line TLiand thereby cause the release of relay ill of signal receiver SRI. The operation of relay lll also closes a circuit for operating cut-oi! relay ill. Relay lli opensthe talking connection between the repeating coil ill and hybrid coil HCi, connects the repeater termination comprising resistor ill and condenser ill across coil ill, connects ground to conductor lll to maintain a busy indication ior trunk circuits 0T2 and TWTi while a signal is being transmitted over line TLi, and closes a circuit for operating relay 3'06. The operation of relay ill opens the operating circuit of relay ill and opens the normally closed shunt circuit through resistor 3l5 across generator lll thereby to increase the voltage applied across conductors ill and Ill when relay ill releases to transmit a signal, for instance an on hook or busy signal. The release of relay 305 disconnects the repeater termination from coil ill, reconnects repeating coil ill to coil H02, and opens.

the operating circuit of relay ill. Relay lll is then operated under the sole control of relay ill.

The release of relay ill responsive to the termination of signaling current over toll line TLi,

when the call is answered by an operator. at position 0P2, causes the release of relays ill and ill;

is routed through selector T82 and a selected idle incoming trunk circuit to a toll operator's position in the second toll ofllce and that the selected In this case, the operation of relay ill of selector TSi closes a circuit for operating line relay ll! of trunk circuit Hi. This circuit is traced from ground at the back contact of relay lll, through conductor 4', lower back contact of relay lll of outgoing trunk circuit OTi. conductor lll, back "and connects ground, to conductor l'll, thereby operating supervisory relay I05 of trunk circuit 'OTi. Relay disconnects ground from' the low resistance winding of sleeve relay Ill thereby causing the calling supervisory lamp CS to be extinguished to indicate that the call has been answered at the called toll oillce. When the answering operator disconnects cord CD2 from jack hook signal. The aforementioned release of relay 300 also causes the operation of relay Ill and the release of relay 300. The release of relay causes the release of relay I and again closes the shunt circuit through resistor II 5 across generator I I0, whereby the voltage applied across conductors III and III is reduced and the signaling current transmitted over line TL! is reduced to the normal, idle condition.

If a busy condition is encountered in extending the connection, for instance, if selector TS! finds all trunks busy in the selected level, relay III of two-way trunk circuit TWT2 is alternately operated and released under the control of an interrupter (not shown), thereby causing the alternate operation and release of relay 300 of signal transmitter 8T2. Relay 300 alternately disconnects and reconnects generator 3l0 to coil ill so that signaling current is intermittently transmitted over line TL2 to cause the alternate release and reoperation of relay 200 of signal receiver SRI.

The alternate release and reoperation of relay 200 cause the alternate operation and release of supervisory relay I 05 of trunk circuit OTI whereby the supervisory lamp CS of cord CDI is intermittently extinguished and lighted thus giving the calling operator a busy signal.

When the calling operator disconnects plug CPI from jack J I,relay I04 releases; but, if relay I05 is operated at this time, relays H0 and I40 are held operated. With relay I05 released, or as soon after the release of relay I04 as relay I05 releases, relays H0 and I40 were also released. The release of relay I40 causes therelease of ,relay 200 whereby generator 2I0 is reconnected across conductors 2|! and 2I0 thereby to effect the transmission of signaling current over line TLI to effect the reoperation of relay 350 of signal receiver SR2. The release of relay 100 also causes the operation of relay 105 and the release of relay 205. The operation of relay 205 causes the release of relay 206 and closes the shunt circuit through resistor 2l5 across generator 2|0 whereby the voltage applied to conductors 2| I and M0 is reduced to the normal level. The operation of relay 390 disconnects ground from conductor 410 causing the release of relay 505 of the trunk circuit TWTI and line relay 005 of trunk circuit 1T2. The continued release of relay 505 causes the release of relays 501 and 503 of trunk circuit TWTI and the release of relay 560 of selector T82 whereby selector TS2 is restored to normal in usual and well-known manner. The release of relay 503 also disconnects ground from conductor 400 thereby terminating the busy marking of trunk circuits OT! and TWT2.

What is claimed is:

1. In a signaling system in which current of a particular frequency within the voice frequency range is used to effect the transmission of signals, a signal receiver, means for at times transmitting voice currents and/or signaling current to said receiver, a voltage dividing network in said receiver comprising a parallel resonant circuit and a series resonant circuit connected in series, both of said resonant circuits tuned to said particular frequency, means including volume lim iting means in said receiver for applying across said resonant circuits in series voltages derived from the voice and signaling currents incoming to said receiver, a signal relay having two windings, rectifying means for applying the voltage across the parallel resonant circuit to energize e winding of said relay and the voltage across 10 the series resonant circuit to energize the other winding of said relay, said'windings being diilerentially energized, and uni-directional means shunting said other winding to permit operation of said relay only when the current incoming to predominantly of said parseries forming a series resonant circuit, said resonant circuits tuned to resonance at said particular frequency, means including volume limiting means for applying across said resonant circuits in series voltages derived from said input circuit, a signal relay, rectifier means for applying the voltages developed across said parallel resonant circuit to one winding of said relay, rectifier means for applying the voltages developed across said series resonant circuit to another winding of said relay, the energization of said other winding opposing the energization of said 'one winding, and uni-directional means shunting said other winding, whereby said relay is operatively energized when signaling current of said particular frequency is being received in said input circuit and is not energizedwhen current of frequencies other than said signaling frequency predominates in said input circuit.

3. In a signal receiver, an input circuit for receiving at times voice currents and/or signaling current of a particular frequency within the voice frequency range, an inductor and capacitor connected in parallel forming a parallel resonant circuit, an inductor and capacitor connected in series forming a series resonant circ'uit, said resonant circuits tuned to resonance at said particular frequency, means including volume limiting means for applying across said resonant circuits in series voltages derived from said input circuit, a signal relay, rectifier means for applying the voltages developed across said parallel resonant circuit to one winding of said relay, rectifier means for applying the voltages developed across said series resonant circuit to another winding of said relay, the energization of said other winding opposing the energization of said one winding, uni-directional means shunting said other winding, whereby said relay is operatively energized only when signaling current of said particular frequency is being received in said input circuit, and means rendered effective by the operation of said relay toapply the voltages derived from said input circuit through the first mentioned rectifier means to energize only said one winding of said relay.

4. In a signal receiver, an input circuit for receiving at times voice currents and/or signaling current of a particular frequency within the voice frequency range, an inductor and capacitor connected in parallel forming a parallel resonant circuit, an inductor and capacitor connected in series forming a series resonant circuit, said resonant circuits tuned to resonance at said particular frequency, means including volume limiting means for applyin across said resonant circuits in series voltages derived from said input circuit, a signal relay, rectifier means for applying the voltages developed across said parallel resonant circuit to one winding of said relay, rectifier means for applying the voltages developedacross said series resonant circuit to another winding of said relay. the energization of said other winding opposing the energization of said one winding,

uni-directional means shunting said other winding, whereby said relay is operatively energized said series tuned circuit, and for applying the entire voltage derived from said input circuit through the first mentioned rectifier means to energize only said one winding of said signal relay.

5. In a multioflice telephone system, a trunk line interconnecting a first and a second oflice, means in-said first oflice for at times transmitting over said line voice currents and/or signaling current of a particular frequency within the voice frequency range, a talkin channel in said second omc'e for transmitting voice currents incoming over said line, a signal receiver in said second oflic'e connected to said line, said signal receivercomprising a signal relay and means for operating "12 a said relay responsiveto signaling current incoming over said line'and for preventing the operation of said relay responsive to voice currents incoming over said line, an inductor and capacitor connected in parallel forming a parallel resqnant circuit. an inductor and capacitor connected in series forming a series resonant circuit, said resonant circuits tuned to said particular frequency, and means for connecting said parallel resonant circuit in series in said talking channel and for bridging said series resonant circuit across said talking channel while said signal relay is operated.

V NORMAN A.

REFERENCES cr ran The following references are of record in the file of this patent:

V UNITED PATENTS Number Name Date 2,209,777 King et al July 30, 1940 2,306,236

Walsh Dec. 22, 1942 

